Inkjet printing mechanisms use moveable cartridges, also called pens, that use one or more printheads formed with very small orifices (also called nozzles) through which drops of liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent) are fired. To print an image, the carriage traverses over the surface of the print medium, and the ink ejection elements associated with the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The pattern of pixels on the print media resulting from the firing of ink drops results in the printed image.
In thermal inkjet printing, electrical resistance heating is used to vaporize ink. The vaporized ink produces a bubble that acts as a piston to expel ink through an orifice in the inkjet pen toward the print medium. Each orifice is associated with an electrical heating resistor. When an electrical heating resistor is electrically energized, ink droplets are vaporized and ejected from an ink chamber associated with the resistor and orifice. A microprocessor selects the appropriate resistors to be fired and directs an electrical current thereto to achieve resistive heating and consequential ejection of ink through the orifice associated with the selected resistor.
In order to determine the optimal firing energy for an inkjet printhead, the printer executes a thermal turn-on energy (TTOE) test. During the test the printhead is fired over a range of print energies while simultaneously monitoring the printhead temperature. The optimal firing energy has been empirically determined to be the printhead's turn-on energy (TOE) plus a fixed percentage (over-energy) to provide margin. Although the best way to determine the TOE is by measuring drop weights, it can be approximated by measuring the temperature of the printhead silicon while firing multiple drops from the printhead. The printhead is fired at discrete steps of firing energy, and the temperature is measured at each step. In this way, the relationship between firing energy and printhead temperature is determined. The thermal TOE is considered to occur when the printhead temperature as a function of firing energy is at or near a local minimum. See, for example, U.S. Pat. No. 6,474,772 B1 issued to Kawamura et al. for a “Method of Determining Thermal Turn on Energy”.
For example, the test determines TOE by holding the firing voltage constant, while firing the printhead for a sustained period and monitoring the printhead temperature. This process begins with a high value for the firing pulse width, and then is repeated for progressively smaller pulse width values. When the test detects that the local temperature minimum has been reached, the pulse width value is saved and noted as the “turn on energy” of that particular inkjet printhead.